Various types of control systems to control lifting apparatus as well as the pulling effort in agricultural machinery are described in the literature reference "Landtechnik" ("Agricultural Technology", April 1974, page 151 et seq.). In accordance with a principle thereof, a mixed control mode is possible in which transducers applied to the tractor unit sense the pulling force applied by the tractor; additionally, transducers sense the position of the drawn element. Of course, plough-and-tractor combinations may be a single element, in which the pulling force is measured, as well as the penetrating depth, for example the depth of furrows.
The control unit combines the sensed values and controls the pulling force as well as the position of the pulled agricultural implement, for example the plough. The positioning control is used solely to control the position of the pulled element with respect to the tractor element; by mixing the signals, deviations in the desired depth of penetration due to control of the pulling effort can be decreased.
The sensed actual values of implement position and pulling force are determined entirely independently of each other. They should, however, be compared with a single command level which corresponds to a certain mixture of pulling force and position. Accordingly, in a mixed control unit, change of the mixing values results in an undesired change in position of the pulled implement. The reason is, of course, that the actual values derived from the respective transducers have no relationship to each other at all. The operator of a tractor thus had to re-adjust the command of the position each time the mixed force command, that is, the command which both combines and relates position and pulling force is re-adjusted.